security/loadpin/loadpin.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Module and Firmware Pinning Security Module
   4  *
   5  * Copyright 2011-2016 Google Inc.
   6  *
   7  * Author: Kees Cook <keescook@chromium.org>
   8  */
   9
  10 #define pr_fmt(fmt) "LoadPin: " fmt
  11
  12 #include <linux/module.h>
  13 #include <linux/fs.h>
  14 #include <linux/lsm_hooks.h>
  15 #include <linux/mount.h>
  16 #include <linux/blkdev.h>
  17 #include <linux/path.h>
  18 #include <linux/sched.h>        /* current */
  19 #include <linux/string_helpers.h>
  20
  21 static void report_load(const char *origin, struct file *file, char *operation)
  22 {
  23         char *cmdline, *pathname;
  24
  25         pathname = kstrdup_quotable_file(file, GFP_KERNEL);
  26         cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
  27
  28         pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
  29                   origin, operation,
  30                   (pathname && pathname[0] != '<') ? "\"" : "",
  31                   pathname,
  32                   (pathname && pathname[0] != '<') ? "\"" : "",
  33                   task_pid_nr(current),
  34                   cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
  35
  36         kfree(cmdline);
  37         kfree(pathname);
  38 }
  39
  40 static int enforce = IS_ENABLED(CONFIG_SECURITY_LOADPIN_ENFORCE);
  41 static char *exclude_read_files[READING_MAX_ID];
  42 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
  43 static struct super_block *pinned_root;
  44 static DEFINE_SPINLOCK(pinned_root_spinlock);
  45
  46 #ifdef CONFIG_SYSCTL
  47
  48 static struct ctl_path loadpin_sysctl_path[] = {
  49         { .procname = "kernel", },
  50         { .procname = "loadpin", },
  51         { }
  52 };
  53
  54 static struct ctl_table loadpin_sysctl_table[] = {
  55         {
  56                 .procname       = "enforce",
  57                 .data           = &enforce,
  58                 .maxlen         = sizeof(int),
  59                 .mode           = 0644,
  60                 .proc_handler   = proc_dointvec_minmax,
  61                 .extra1         = SYSCTL_ZERO,
  62                 .extra2         = SYSCTL_ONE,
  63         },
  64         { }
  65 };
  66
  67 /*
  68  * This must be called after early kernel init, since then the rootdev
  69  * is available.
  70  */
  71 static void check_pinning_enforcement(struct super_block *mnt_sb)
  72 {
  73         bool ro = false;
  74
  75         /*
  76          * If load pinning is not enforced via a read-only block
  77          * device, allow sysctl to change modes for testing.
  78          */
  79         if (mnt_sb->s_bdev) {
  80                 char bdev[BDEVNAME_SIZE];
  81
  82                 ro = bdev_read_only(mnt_sb->s_bdev);
  83                 bdevname(mnt_sb->s_bdev, bdev);
  84                 pr_info("%s (%u:%u): %s\n", bdev,
  85                         MAJOR(mnt_sb->s_bdev->bd_dev),
  86                         MINOR(mnt_sb->s_bdev->bd_dev),
  87                         ro ? "read-only" : "writable");
  88         } else
  89                 pr_info("mnt_sb lacks block device, treating as: writable\n");
  90
  91         if (!ro) {
  92                 if (!register_sysctl_paths(loadpin_sysctl_path,
  93                                            loadpin_sysctl_table))
  94                         pr_notice("sysctl registration failed!\n");
  95                 else
  96                         pr_info("enforcement can be disabled.\n");
  97         } else
  98                 pr_info("load pinning engaged.\n");
  99 }
 100 #else
 101 static void check_pinning_enforcement(struct super_block *mnt_sb)
 102 {
 103         pr_info("load pinning engaged.\n");
 104 }
 105 #endif
 106
 107 static void loadpin_sb_free_security(struct super_block *mnt_sb)
 108 {
 109         /*
 110          * When unmounting the filesystem we were using for load
 111          * pinning, we acknowledge the superblock release, but make sure
 112          * no other modules or firmware can be loaded.
 113          */
 114         if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
 115                 pinned_root = ERR_PTR(-EIO);
 116                 pr_info("umount pinned fs: refusing further loads\n");
 117         }
 118 }
 119
 120 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
 121 {
 122         struct super_block *load_root;
 123         const char *origin = kernel_read_file_id_str(id);
 124
 125         /* If the file id is excluded, ignore the pinning. */
 126         if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 127             ignore_read_file_id[id]) {
 128                 report_load(origin, file, "pinning-excluded");
 129                 return 0;
 130         }
 131
 132         /* This handles the older init_module API that has a NULL file. */
 133         if (!file) {
 134                 if (!enforce) {
 135                         report_load(origin, NULL, "old-api-pinning-ignored");
 136                         return 0;
 137                 }
 138
 139                 report_load(origin, NULL, "old-api-denied");
 140                 return -EPERM;
 141         }
 142
 143         load_root = file->f_path.mnt->mnt_sb;
 144
 145         /* First loaded module/firmware defines the root for all others. */
 146         spin_lock(&pinned_root_spinlock);
 147         /*
 148          * pinned_root is only NULL at startup. Otherwise, it is either
 149          * a valid reference, or an ERR_PTR.
 150          */
 151         if (!pinned_root) {
 152                 pinned_root = load_root;
 153                 /*
 154                  * Unlock now since it's only pinned_root we care about.
 155                  * In the worst case, we will (correctly) report pinning
 156                  * failures before we have announced that pinning is
 157                  * enforcing. This would be purely cosmetic.
 158                  */
 159                 spin_unlock(&pinned_root_spinlock);
 160                 check_pinning_enforcement(pinned_root);
 161                 report_load(origin, file, "pinned");
 162         } else {
 163                 spin_unlock(&pinned_root_spinlock);
 164         }
 165
 166         if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
 167                 if (unlikely(!enforce)) {
 168                         report_load(origin, file, "pinning-ignored");
 169                         return 0;
 170                 }
 171
 172                 report_load(origin, file, "denied");
 173                 return -EPERM;
 174         }
 175
 176         return 0;
 177 }
 178
 179 static int loadpin_load_data(enum kernel_load_data_id id)
 180 {
 181         return loadpin_read_file(NULL, (enum kernel_read_file_id) id);
 182 }
 183
 184 static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
 185         LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
 186         LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
 187         LSM_HOOK_INIT(kernel_load_data, loadpin_load_data),
 188 };
 189
 190 static void __init parse_exclude(void)
 191 {
 192         int i, j;
 193         char *cur;
 194
 195         /*
 196          * Make sure all the arrays stay within expected sizes. This
 197          * is slightly weird because kernel_read_file_str[] includes
 198          * READING_MAX_ID, which isn't actually meaningful here.
 199          */
 200         BUILD_BUG_ON(ARRAY_SIZE(exclude_read_files) !=
 201                      ARRAY_SIZE(ignore_read_file_id));
 202         BUILD_BUG_ON(ARRAY_SIZE(kernel_read_file_str) <
 203                      ARRAY_SIZE(ignore_read_file_id));
 204
 205         for (i = 0; i < ARRAY_SIZE(exclude_read_files); i++) {
 206                 cur = exclude_read_files[i];
 207                 if (!cur)
 208                         break;
 209                 if (*cur == '\0')
 210                         continue;
 211
 212                 for (j = 0; j < ARRAY_SIZE(ignore_read_file_id); j++) {
 213                         if (strcmp(cur, kernel_read_file_str[j]) == 0) {
 214                                 pr_info("excluding: %s\n",
 215                                         kernel_read_file_str[j]);
 216                                 ignore_read_file_id[j] = 1;
 217                                 /*
 218                                  * Can not break, because one read_file_str
 219                                  * may map to more than on read_file_id.
 220                                  */
 221                         }
 222                 }
 223         }
 224 }
 225
 226 static int __init loadpin_init(void)
 227 {
 228         pr_info("ready to pin (currently %senforcing)\n",
 229                 enforce ? "" : "not ");
 230         parse_exclude();
 231         security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
 232         return 0;
 233 }
 234
 235 DEFINE_LSM(loadpin) = {
 236         .name = "loadpin",
 237         .init = loadpin_init,
 238 };
 239
 240 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 241 module_param(enforce, int, 0);
 242 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");
 243 module_param_array_named(exclude, exclude_read_files, charp, NULL, 0);
 244 MODULE_PARM_DESC(exclude, "Exclude pinning specific read file types");